Process for regenerating caustic alkali solutions



Jam. 20, 194-2- D. L. YABROFF ETAL 2,270,491

' PROCESS OF REGENERATING CAUSTIC ALKALI soLUTIoNs Filed Feb. 15, 1959Solufion N N v N 4" 5" I 2 F I 3 T'feafeti Minute Fresh Solvent Z] Hewl'F fl-"g Span! solvent Extractor L. Phen ls Fig. 2.

lnven'fi'ors: David Louis Yubr'oh Ell is l2. Whi'l'e Patented Jan. 20,1942 PROCESS FOR REGENERATING CAUSTIC ALKALI SOLUTIONS David LouisYabrofl, Berkeley, and Ellis R. White, Albany, Calif., assignors toShell Development Company, San Francisco, Calilt, a corporation ofDelaware Application February 13, 1939, Serial No. 256,180

6 Claims.

This invention relates to the removal of weakly acidic organic compoundsfrom aqueous caustic alkali solutions containing same and substantialamounts of solubility promoters for said compounds, and moreparticularly relates to a process oils or may be produced by cracking,e. g. mercaptans, phenols, thiophenols, alkyl phenols, etc. from aqueouscaustic alkali solutions, particularly those caustic alkali solutionswhich have been used for the extraction of the acidic compounds fromhydrocarbon oils and which contain substantial amounts of solubilitypromoters for said acidic compounds.

As described in our U. S. Patents Nos. 2,152,720; 2,152,166; 2,149,379;2,160,632; 2,164,851; 2,152,- 722, we have found that certainsubstances, when added to aqueous caustic alkali treating solutions insubstantial amounts, greatly enhance the solubility of organic acidicsubstances therein.

It has been shown that solubility promoters to be suitable must combinethe following properties: high specific solubility promoting power, 1.e., ability to raise the solvent power of the aqueous caustic alkali forthe free organic acids by a relatively large amount for a unit weightad- 2 examples being propylene glycol, butylene glycols, alkylglycerines in which the alkyl radical has from 2 to 4 carbon atoms, monoalkyl glycerine ethers in which the alkyl radical has 1 or 2 carbonatoms, di-- and tri-ethylene glycol, beta amino beta hydroxy diethylether, diand tri-ethylene glycol mono alkyl ethers in which the alkylradical has'from 1 to 3 carbon atoms; diamino alcohols of 3 to 5 carbonatoms, etc. Very effective are also the alkali metal salts of alkylphenols and of certain carboxylic acids, such as fatty acids having 2 to6 carbon atoms, notably potassium isobutyrate, or of amino or hydroxyfatty acids having 3 to 7 carbon atoms, especially potassium alphahydroxy n-butyrate; or of phenylacetic acids, etc.

The amounts of solubility promoters normally employed in solution of theaqueous caustic alkali vary between the approximate limits of to 85%,and preferably to 75% in the case of neutral and basic compounds. In thecase of salts, the aqueous caustic alkali is preferably sub stantiallysaturated therewith, or at least nearly so.

Various caustic alkalis may be used. While alkali metal hydroxides arepreferred, ammonia, quaternary ammonia bases, alkali metal carbonates,etc., may also be suitable.

The efliciency of the extraction of the organic acids from theirsolutions in organic water-insoluble soluble solvents by means ofaqueous caustic alkalis has been expressed by the factor K. in

which concentration of salts of the organic acid in aqueous phaseconcentration of free organic acids in organic liquid phase dition ofthe promoter to the aqueous causticalkali; high solubility of thepromoter in the aqueous caustic alkali; and very low solubility of thepromoter in the hydrocarbon or other organic water-insoluble liquidcontaining the organic acids. Moreover, the promoter must be chemicallyand physically inert to the action of caustic alkalis even at theelevated temperatures of steam stripping. r l Among the many substanceswhich combine the above properties in varying degrees, the followingwere found most suitable: mono-, diand tri-amino or hydroxy alkyl aminesin which the alkyl groups contain 2 or 3 carbon atoms; diamino propanol;polyhydric alcohols or derivatives thereof having at least 1 carbon atomin excess of polar radicals, a-ratio of carbon atoms to hydroxylradicals of at least 1 and a ratio of carbon atoms to polar radicals notin excessor 55 by first diluting the solutions with water and It may,therefore, be seen that when it is desired to do the opposite, i. e., toremove weak organicacids from their caustic alkali aqueous solutions byscrubbing with an organic solvent for the acids, the most efficientremoval may occur when the caustic alkali solution is diluted, i. e.,when the value of K is low.

Our invention thus comprises removing weak organic acids, particularlythose having dissociation constants below about 10- from aqueoussolutions of caustic alkali containing said acids and substantialamounts of solubility promoter,

thereafter scrubbing the resulting diluted solutions with a solvent fororganic acids.

Solvents to be suitable should be substantially immiscible with theaqueous caustic alkali solutions, and should be chemically inert to theaction of the caustic alkali under the conditions of the extraction.

Suitable solvents are, for example, hydrocarbon distillates such asgasoline, kerosene; substantially pure liquid hydrocarbons as pentane,hexane, benzene, toluene, xylene; chlorinated hydrocarbons as ethylenedichloride, chlorpropane; higher alcohols as octyl alcohols; ethers asdiisopropyl ether, dibutyl ethers, etc. While many polar solvents suchas diisopropyl ether, etc., may be better solvents for the organic acidsthan hydrocarbons and may even dissolve some salts of the organic acids,we often prefer to use the latter because of certain advantages whichare disclosed later.

The above-mentioned effect of dilution on the K value is more fullyexplained in connection with Fig. 1 of the drawing. In Fig. 1' a co-)rdinate system is shown in which K is plotted m the ordinate againstthe normalities of caustic 0d8 solutions of various degrees of dilution,when :xtracting normal butyl mercaptan therewith- :rom a gasoline. Threecases may be distinmished as indicated by curves l, 2 and 3, re-:pectively. v

Curve I shows the change of K for the normal iutyl mercaptan withconcentration of the cansic soda solution in the absence of a solubility)romoter. It begins at about zero value for K, ises approximately as astraight line over a hort distance and then drops'ofi with increasngnormality of the caustic soda solution; This lrop in the value of K inspite of increasing alkainity of the aqueous solution beyond a certainvoint is due to the salting out of the free meraptan by the dissolvedcaustic soda in the aqueus solution, the free mercaptan being inequilibium with the dissolved mercaptide.

Curve 2 represents the change of K with hanging concentration ordilution of a caustic oda solution containing two parts by weight ofodium isobutyrate foreach part of caustic soda. )bviously, the ratio ofcaustic soda to isobutyrate emains the same over the entire range ofconentrations up to the point of saturation of one f the dissolvedcomponents. This curve begins ke curve i at about zero value for K, butinstead f dropping back, turns strongly upward. In ther words, thepresence of the isobutyrate comletely overcomes the salting out effect,and with icreasing alkalinity of the solution and a proortionateincrease in the isobutyrate concentraon, K increases at a greater ratethan is proortional to the increase of the normality of the rustic sodasolution.

Curve 3 represents the boundary case in which 1e caustic soda solutioncontains the minimum mount of solubility promoter necessary to overimethe salting out effect and is therefore es- :ntially a straight line.

Now when attempting to improve the removal r scrubbing of weak organic.acids from their lution in caustic alkali by the expedient of dition,another factor besides the value of K ust be taken into consideration,namely, the

diluting it with an equal volume of water, no increased removalefficiency of the organic acids will result unless the K value of theseacids is reduced to less than half of its former value.

Referring again to curve I, it will be seen that in the low normalityrange below about 1N, dilution has the effect of increasing the volumeand lowering the K value in approximate inverse proportion. In the rangeof higher normality, however, the change in K is less than inverselyproportional to the increase in volume,.the latter resulting, within acertain range, in an increase in K. For example, K for a 3N caustic sodasolution in the absence of a solubility promoter is 7.2, as indicated bypoint A1. If now this caustic solution is diluted with an equal volumeof water, the volume of the solution is doubled, but the K is reducedonly from 7.2 to 6.5, the latter point being indicated by B1. This meansthat the scrubbing efliciency on the basis of equal volumes of causticsoda solution is improved by the factor but on the basis of actualvolumes the net efficiency is'lowered by the factor 0.55. If we nowconsider points A: and Be on curve 2, it will be seen that K is loweredfrom 25 to 9.2 upon diluting the caustic soda solution with an equalvolume of water. This will result in a net improvement of the scrubbingefficiency based on actual volumes of The above comparison clearly showsthat dilution for the'purpose of removing weakly acidic organicsubstances from alkaline solutions of creased volume of the aqueoussolution. In

their salts by scrubbing with an organic solvent for the acids isbeneficial for all solutions which contain amounts of solubilitypromoters at least suflicient to compensate for the salting out of thefree acids from their aqueous solutions by the caustic alkali.

A further increase in removal efliciency may be obtained by raising thetemperature at which the acidic substances are extracted from thecaustic aqueous alkali solution. The temperature of the removal,therefore, is preferably as high as practical, and. is limited only bythe boiling temperature of the solutions and/or solvents used in thistreatment. Superatmospheric pressure may be employed so that theextraction may be carried out at a temperature higher than the normalboiling temperatures of any of the solutions. Preferably the temperatureis over C.

To give a specific example of the way in which the present invention canbe utilized, reference is had to our copending patent application SerialNo. 174,512, filed November 15, 1937. In that application it was shownthat in extracting mercaptans from hydrocarbon distillates with causticalkali solutions containing solubility promoters for mercaptans, thespent caustic alkali solution may be regenerated by steam stripping mosteffectively if the solution is diluted with water before being steamed.However, in that process it is frequently desirable to extracthydrocarbon solutions, which contain alkyl phenols as well asmercaptans. Since alkyl phenols are set free only to a small degree bysteaming, it has been found that alkyl phenols remain in the causticalkali treating solution, and build up upon recirculation to anundesirable degree, an exthrough lines 22 and 6 to the stripping columnS cess of alkyl phenols causing the treating solution to becomeexcessively viscous. However, by combining the two inventions, it hasbeen found that hydrocarbons containing alkyl phenols can be treatedwith caustic alkali solutions, which can be regenerated with steamfollowed by a solvent extraction process, so that alkyl phenols do notunduly build up in the treating solutions.

The above example is illustrated in Fig. 2, reppresenting a flow diagramof a regenerative extraction process for weak organic acids frombydrocarbon distillates, which diagram includes our invention ofscrubbing the spent caustic alkali solution with a solvent-for theacids, preceded by dilution with water.

A hydrocarbon distillate, which may contain both mercaptans and alkylphenols, is introduced near the bottom of extractor E1 through line I,E1 being of conventional design adapted to the contacting of twosubstantially immiscible liquids and preferably containing packing orother means of effecting intimate contact between the two liquids. Acaustic alkali solution containing suflicient solubility promoter atleast to compensate for the salting out of mercaptans and alkyl phenolsby the dissolved caustic alkali is introduced near the top of extractorE1 through line 2. Treated hydrocarbon distillate being at leastpartially freed of mercaptans and alkyl phenols,

is removed from the top of E1 through line 3. The foul caustic alkalisolution, which now contains salts of mercaptans and alkyl phenols, isremoved from E1 through line 4. Water of dilution may be added throughline 5, and the resulting diluted solution is introduced into a columnS, preferably under superatmospheric pressure. Steam is admittedinto-the column S through line 6, either from an outside source notshown or from within the process as will be explained later, and themercaptans, along with the steam and a small amount of the alkyl phenolsare taken overhead through line I, are condensed by condenser 8 andintroduced into separator 9. In this separator, two layers are formed,an upper oily layer containing the mercaptans and those alkyl phenolswhich were taken overhead, and a lower aqueous layer. The upper layer iswithdrawn through line Ill and the lower layer may be reintroduced aswater of dilution into the foul caustic alkali solution through line 5,or ,may be discarded. The caustic alkali solution, which is nowsubstantially free of mercaptans but which may be diluted by thecondensed steam used for steaming ofl the mercaptans is removed from Fthrough line I I. If it is undiluted or insufficiently diluted, water ofdilution is added through line I: and the diluted solution is introducedinto scrubber-extractor E2 near its top, E2 being similar in design toE1. A solvent for alkyl phenols, which may or may not be the samedistillate which was treated in extractor E1, is introduced through linel3 near the bottom of the scrubberextractor E2 to scrub out as much aspossible the alkyl phenols and other remaining weak acids from thecaustic soda solution. Spent solvent which contains a substantialportion of the alkyl phenols is removed from the top of the extractor E:through line H. The scrubbed dilute caustic alkali solution, now havingsubstantially reduced contents of both the mercaptans and alkyl phenols,is withdrawn through line l5, and is reconcentrated in boiler- IE to itsoriginal strength, to be recirculated through line 2 into extractor E1.Steam produced in the boiler may bewithdrawn through line 2| or may bereturned to be used in the stripping of the mercaptans. Since theregenerated caustic alkali solution is hot as it leaves boiler I6, itcan advantageously be placed in heat exchanging relationship with thedistillate entering extractor E2 by means of heat exchanger I'I, since,as pointed out above, increasing the temperature in the scrubbing steplowers the value of K which governs the efficiency of this step. Alkylphenols being valuable'as gum inhibitors for the distillate, thedistillate originally containing the 'alkyl phenols may be usedadvantageously for scrubbing the dilute caustic alkali solution.However, if desired, a different solvent may be used, provisionbeingmade for its introduction through line l8 and for the withdrawal ofthe distillate treated in E1 through line l9. As an alternative of theabove process, if it is not desired to include a steam strippingoperation, the spent caustic alkali solution from extractor E1 may beby-passed through lines 20 and Il directly to the top of thescrubber-extractor E2.

While in the foregoing we have described a preferred form of ourprocess, it shall be understood that many modiflcations are within thescope of our invention. In particular, it shall be understood that theaddition of pumps, coolers, heat exchangers, heaters, by-passes, etc.,not shown in the flow diagram, is within the skill of the averagedesigner for such equipment.

The following example further illustrates our invention:

A sample of an aqueous 6N KOH solution containing 135 grams per literpotassium isobutyrate and 135 grams per liter alkyl phenolate wasscrubbed with 10 volumes of a gasoline. The amount of alkyl phenolsdissolved in the gasoline after the scrubbing treatment was estimated bymeasuring the oxygen bomb induction period of the resulting gasoline,which period was 4.0 hours.

Another sample of the same aqueous KOH solution was then diluted with anequal amount of water, and the resulting dilute solution was scrubbedwith 5 volumes of the same gasoline,

stants below 10-, are extracted from hydrocarbon oils containing themwith an aqueous solution of caustic alkali containing dissolved thereinalkali metal saltsof fatty acids having 2 tot carbon atoms in an amount'sufiicient at least to compensate for salting out of said weak acidsfrom the aqueous caustic alkali solution, to produce a treated oil and afat aqueous solution, the latter containing absorbed said weakly acidicorganic substances, and wherein said fat solution is regenerated byscrubbing to produce a regenerated aqueous solution containingsubstantially all of said solubility promoter but having a reducedcontent of said organic acids, the improvement comprising diluting saidfat solution with water and thereafter scrubbing the diluted solutionwith an organic solvent for said weakly acidic substances, which solventis insoluble in and inert to said caustic alkali solutions underconditions of scrubbing, thereby removing from the fat solution weaklyacidic substances having dissociation constants below 10" withoutremoving fatty acids.

ing point of either the solvent or the caustic 2. The process of clairn1 in Which the scrubbing is carried out at a temperature above60 F'.

' but below the boiling temperatures of the solvent and the causticalkalisolution.

3. The process of claim 1 in which the extraction is carried out undersuperatmospheric pressure at temperatures higher than the normalboilalkali solution.

4. The process of claim 1 in which the solvent

